1. Field of the Invention
The invention relates generally to refrigerated display cases, and in particular to the defrosting of such cases by combining hot gas defrost, which is to say utilization of the working fluid of the refrigeration system as a basic warming means, with the circulation of air to which heat has been transferred from the working fluid, over problem areas characterized by their high resistance to defrost.
2. Description of the Prior Art
Heretofore, in refrigerated display cases utilizing hot gas for defrost purposes, the practice has been to operate the conventional air circulating evaporator fans, during a defrost cycle. When the fans are operated during defrost, conventionally they circulate air through the duct or air passage of the case in the same direction as during the refrigeration mode. These areas, in particular the tank drain, are located conventionally on the return air side of the evaporator coil, so as to keep the drain at the highest possible temperature during a refrigeration cycle. So locating it, however, has an effect opposite from that desired, during a defrost cycle, because air conventionally circulated by the fans and warmed by the defrosting evaporator coil gives up a substantial part of its heat during its circulation through the same path and in the same direction as it travels during the refrigeration mode. The return air flue and the tank drain thus, during a defrost cycle, become the last to receive the warming effect of the circulated air in these circumstances.
Accordingly, it has been conventional practice to install additional tubing on many cases, the design of which renders the return air flue and tank drain areas thereof particularly difficult to defrost. This tubing is in effect a continuation of the tubing or piping through which the refrigerant fluid (and hence the hot gas) flows. The tubing is conventionally extended as a "warming loop" around the exterior of the evaporator coil and within the vicinity of the tank drain area. Thus, hot gas which would normally flow directly into the evaporator coil during the defrost cycle, is caused to first travel through the loop and over or around the tank drain, for transfer of heat directly from the loop to the coil and drain. The drain is thus defrosted by said heat transfer from the hot gas. The hot gas thereafter passes directly into the evaporator coil to accomplish the desired defrost thereof in a known manner.
The procedure of installing additional tubing, as warming loops, is a costly one. If not installed properly, it can result in icing in the tank and drain. Even so, the presence of a warming loop still has no effect on the return air flue, which being the last to receive the circulated air warmed by defrost of the coil, tends to be the slowest to reach a fully defrosted condition.